Dave Ansell, Cambridge Science Centre
This week, NASA unveiled plans to set fire to an (unmanned) space station, because we have relatively poor understanding of how fires behave in space. Dave Ansell gave Chris Smith further detail on the story...
Dave - Well the problem is that fire on earth we understand very well. It works because air heats up, expands and then it floats upwards and that brings more air in underneath, and the flames go upwards and the air comes into the bottom and it burns away happily and we understand that really, really well. In space basically, itís microgravity which means that everything's falling, so heavy things donít fall faster than light things which means light things donít float up above heavy things. So fire in space behaves completely differently. Itís entirely dependent on the rate at which oxygen can diffuse into the fuel and that also means it gets cooled down less quickly because the air rushing past it also cools it down and, to be honest, they just donít know what works and they donít know how, especially, a big fire will happen because, luckily, thereís never been one on a spaceship.
Chris - So just in summary then. A candle on earth you light that, this will produce a chemical reaction that produces some heat which causes the hot gases, which are the combustion products (a lot of carbon dioxide), theyíre going to rise up out of the way pulling in nice fresh air with lots of oxygen, and thatís because the candle ďknowsĒ where up and down is?
Dave - Yes.
Chris - Because of earthís gravity. If youíre in freefall in space, thereís no net up and down so the gas blows up just like a bubble, the exhaust gases and this would choke off the flame, one would think.
Dave - Because itís not getting cooled down as quickly it still can burn. It burns much more slowly and itís much less predictable because thereís not up no down, you donít know what direction itís going to travel.
Chris - So what are NASA going to do?
Dave - So luckily, the cargo ships which theyíre sending up are one of the kind of ships which have an oxygen atmosphere because most spaceships donít have an oxygen atmosphere. Thereís no people there: who cares? They go up to the space station, they unload a load of kit and then they just go down and get burned up in the atmosphere. So someone thought ha ha, we can use this. Letís put a big box inside with some stuff to burn on the way back down. In a couple of months time on its way back down theyíre going to set fire to it and see what happens. They donít think itís actually going to cause the spaceship to be damaged.
Chris - How are they going to monitor it? Have they got cameras in there are something?
Dave - Theyíve got cameras in there and lots of sensors (gas sensors), temperature sensors, as much as they can possibly get into a small box. Itís going to be a 4ft long by 2ft high box - itís actually quite big for sending out into space and hopefully that means that the next space station that they build, theyíll understand fire a lot better, which means they can build it to cope with fires a lot better because at some point itís going to happen and it could be very, very nasty.
Chris - Sure. Have they got various over-engineered solutions in the present space station just in case there were a fire and thatís obviously adding a lot of weight, and if you could save a lot of weight because you engineer things slightly differently because you donít need it, it would obviously make space travel more predictable and viable ?
Dave - Definitely, when youíve got an uncertainty, engineers will increase the margins and make everything heavier and more expensive and more high tech, and if we can get away without it, or we might find thereís some great big holes in the defences and you could burn out a space station and we donít know that itís going to happen.
Chris Basu - How do people plan to put fires out on space anyway because you canít use sprinklers because the water would just float away?
Chris - The water would just be hanging on with the fire extinguisher wouldnít you?
Dave - I think you can, I guess, for small fires some kind of fire extinguisher will work. Astronauts are trained to stand on things and as long as theyíre expecting it, they can deal with it. With extreme fires you can pump halon gas into the chamber which also will kill any people in there so youíve got to make sure you evacuate first. I guess, if itís really, really extreme you can just vent all the air out of it and there's no oxygen there to burn.